The invention concerns a gradient coil system for the production of a magnetic transverse gradient field G.sub.x =dB.sub.z /dx in a nuclear magnetic resonance (NMR) tomograph or spectrometer with a main field magnet for the production of a homogeneous static main magnetic field B.sub.z in a measuring volume whose center coincides with an origin of a Cartesian x-, y-, z-coordinate system, whereby the main magnetic field B.sub.z is directed along the z-axis and the magnetic transverse gradient field G.sub.x along the x-axis of this coordinate system, whereby the gradient coil system comprises four partial coils (S.sub.1x, S.sub.2x, S.sub.3x, S.sub.4x) each having two current connections (A.sub.1, A.sub.2) which are arranged mirror symmetrically with respect to the xy-plane (z=0) and mirror symmetrically with respect to the zy-plane (x=0), whereby each partial coil contains winding sections on only an inner and an outer cylinder Z.sub.ix, Z.sub.ax extending about the z-axis and, in each case, in a radial connecting plane V.sub.+x, V.sub.-x parallel to the xy-plane, whereby the winding sections of each partial coil have current flowing through them in series during operation and whereby the radial connecting planes V.sub.+x, V.sub.-x of those partial coils (S.sub.1x, S.sub.4x ; S.sub.2x, S.sub.3x) which lie across from each other relative to the zy-plane, are identical.
A gradient coil system of this kind is known in the art from GB 22 65 986 A.
An essential component of NMR systems utilized primarily for tomography but also to a certain extent for spectroscopy, is a system of normally three gradient coils comprising a plurality of partial coils which are to be independently fed with currents of differing strengths. These coils serve the purpose of overlapping the homogeneous magnetic field B.sub.0z of the main field magnet with constant field gradients of adjustable strength, whereby the direction of one of these gradients (dB.sub.z /dz) is normally parallel to the direction of the homogeneous main field B.sub.0z, i.e. to the z-axis (z-gradient=axial gradient) and the direction of the two other gradients (dB.sub.z /dx, dB.sub.z /dy) run orthogonal thereto and to each other transverse to the direction of the main field (x and y-gradients=transverse gradients). The spatial region in which the magnetic field of these gradient coils has a nearly linear dependence can be utilized for spatially resolved NMR methods (imaging, volume selective spectroscopy) to the extent that these regions are not further limited by main field inhomogeneities.
In order to shield the effect of the gradient coils towards the outside, active shielding coils, associated with each partial coil of the gradient coil system, are provided for in the systems known in the art having a larger radial separation from the z-axis than the gradient coils themselves. For example, known in the art from DE 42 10 217 A1 is a transverse gradient coil system, for example to produce an x-gradient G.sub.x which, in addition to the four partial coils for the production of the x-gradients G.sub.x, comprises an additional four partial coils for shielding the gradient coils. The x-gradient coil system known in the art therefore consists of a total of eight partial coils of which the actual gradient coils are arranged on a inner cylinder and the shielding coils on an outer cylinder about the z-axis.
A disadvantage of this conventional gradient coil system is that, for the production of the transverse gradient, only one partial coil region is useful which lies in the vicinity of the xy-plane (z=0). The return sections of the partial coils are, in contrast, not usable or even destructive with regard to the produced transverse gradient linearity. In addition these return sections increase the electrical resistivity as well as the total inductivity and the entire length of the gradient coil system.
An improvement is, in contrast thereto, represented by the gradient coil system in accordance with the above mentioned GB 22 65 986 A. The system presented therein contains only four instead of eight partial coils per gradient device, whereby each coil comprises of two cylindrical sections and one planar section connecting the two sections in a plane perpendicular to the z-axis. The return loops are, in contrast to the configuration in accordance with DE 42 10 217 A1 fed, as it were, in a radially outward plane and close in on themselves on a shielding cylinder of large radius. Towards this end the connecting plane V of the sections of each partial coil is, in each case, located on the portion of the coil facing away from the xy-plane, i.e. at maximum distance from the middle plane.
The gradient coil system in accordance with GB 22 65 986 A has a smaller electrical resistivity, a lower inductivity and a smaller axial extension along the z-axis than, for example, the system described in DE 42 10 217 A1. In addition, this gradient coil system has transverse gradients with better linearity and no "gradient reversal" in the vicinity of return loops is observed.
GB 22 65 986 A however only discloses configurations in which the shielding windings located on the outer cylinder extend from the radially connecting plane V towards the middle plane (z=0). The connecting planes V of both cylinders in which the radial sections of the windings of the corresponding partial coils are located, is therefore at a maximum separation from the xy-plane. A configuration of this type generally shields the effect of the transverse gradient towards the outside in an imperfect manner since, in the region of the corresponding connecting plane V, unshielded stray fields remain which can only be compensated by currents which, as seen from the middle plane (z=0) must flow on the other side of the connecting plane V. This is, however, in the gradient coil configuration in accordance with GB 22 65 986 A, impossible.
It is therefore the purpose of the current invention to present a gradient coil system of the above mentioned kind which exhibits nearly perfect shielding, whereby the advantages of the system known in the art through GB 22 65 986 A compared to, for example, the system known in the art from DE 42 10 217 A1 are, however, preserved.